Typical structures of trench metal-oxide-semiconductor-field-effect-transistors (MOSFETs) having trenched floating gates as termination in prior arts are encountering technical problems. For example, in U.S. Pat. No. 6,462,376, a trench MOSFET was disclosed with n+ source regions disposed in termination area comprising multiple trenched floating gates, as shown in FIG. 1A. In the termination area, a plurality of the n+ source regions 120 are disposed between two adjacent of the trenched floating gates 111. This kind of structure will cause heavily leakage current between drain region and the source region because that channel region is easily turned on in the termination area due to a plurality of P body regions 108 having floating voltage and the trenched floating gates 111 are not shorted together with the n+ source regions 120 at drain/source reversed bias. An electrical current will flow from the drain region through the channel region between two adjacent of the trenched floating gates 111 in the termination area to the n+ source region 120 in active area.
Please refer to FIG. 1B for another prior art U.S. Pat. No. 7,511,339 which disclosed another trench MOSFET structure without having source regions in the termination area but with depth of the trenched floating gates 110 (TFd) shallower than depth of floating deep P body regions 130 (Pd). However, from experimental results of the relationship between breakdown voltage (BV) and difference between TFd and Pd in FIG. 2, it can be seen that, the breakdown voltage is significantly degraded as the difference (TFd−Pd) is getting smaller when TFd<Pd, thus causing low breakdown voltage in the termination area due to poor isolation between drain region and source region by the trenched floating gates 110 having shallower depth in the termination area. The floating deep P body regions 130 are electrically connected together by charge depletion of the floating deep P body regions 130 at drain/source reversed bias because the trenched floating gates 110 is shallower than the floating deep P body regions 130. Therefore, an electrically current will directly flow from edge of the termination area to the source regions 131 in the active area without being blocked by the trenched floating gates 110 in the termination area.
Furthermore, in order to ensure the potential around the device edge has same potential after sawing for uniform breakdown voltage, an equal potential ring (EPR, similarly hereinafter) is normally formed in the termination area of a trench MOSFET surrounding source metal pad and gate metal pad of the trench MOSFET.
Therefore, there is still a need in the art of the semiconductor device design and fabrication, particularly for trench MOSFET design and fabrication, to provide a novel cell structure, device configuration and fabrication process that would resolve these difficulties and design limitations. Specifically, it is desirable to maintain high breakdown voltage in the termination area of a trench MOSFET.